Arcing is a well known problem in AC power switching. Arcing is the creation of an electrical arc between the contacts as they begin to open from a closed position. If, as the contacts open, the voltage across the contacts reaches a sufficient level, an arc will form between the contacts. Furthermore, if an arc does form, the arc may continue even after the contacts are well open. Arcing is well known to be undesirable because of the wear that arcing inflicts on the contacts, and because of undesirable circuit effects caused by arcing.
Protection relays contain circuits with mechanical trip contacts for switching-on and switching off AC power circuit breakers. The mechanical contacts are coupled to switch-on and switch off an “opening solenoid” that is mounted to the circuit breaker. These mechanical contacts are subjected to an inductive DC current load, the load presented by the “opening solenoid” of an AC power circuit breaker. So the contacts of the arc suppression circuits themselves need protection from wear caused by arcing. Increasingly, arc suppression circuits are being used to protect such mechanical contacts. The arc suppression circuits are typically mounted in a protection relay, and are located proximate to the mechanical contacts that they are to protect.
U.S. Pat. Nos. 5,703,743 and 5,652,688 disclose such arc suppression circuits. These patents disclose circuits having a normally-off power transistor with particular operating characteristics. The increase in the voltage across the trip contacts as the contacts open is used as an activating signal to turn on the normally-off power transistor, momentarily shunting the load current around the contacts during the time the contacts are opening.